The invention relates to a microlithography objective, a projection exposure apparatus containing the objective, and a method of manufacturing an integrated circuit using the same.
Using a lithography system operating with wavelengths below 193 nm for imaging structures of below 130 nm resolution has been proposed. In fact, such lithography systems have been suggested for the extreme ultraviolet (EUV) range with wavelengths of xcex=11 nm or xcex=13 nm producing structures of below 100 nm. The resolution of a lithographic system is described by the following equation:
RES=k1xc2x7xcex/NA
where k1 is a specific parameter of the lithographic process, xcex is the wavelength of the incident light, and NA is the image-side numerical aperture of the system. For example, if one assumes a numerical aperture of 0.2, then the imaging of 50 nm structures with 13 nm radiation requires a process with k1=0.77. With k1=0.64, the imaging of 35 nm structures is possible with 11 nm radiation.
For imaging systems in the EUV region, substantially reflective systems with multilayer coatings are available as optical components. Preferably multilayers of Mo/Be are used as multilayer coating systems for systems operating at xcex=11 nm, whereas Mo/Si systems are used for xcex=13 mm. Since the reflectivity of the multilayer coatings is approximating 70%, it is desirable to use as few optical components as possible in e.g. an EUV projection microlithography objective to achieve sufficient light intensity. Specifically, to achieve high light intensity and to allow for the correction of imaging errors, systems with six mirrors and a image side numerical aperture (NA)=0.20 have been used.
Six-mirror systems for microlithography have become known from the publications U.S. Pat. No. 5,686,728, EP 779,528 and U.S. Pat. No. 5,815,310. The projection lithography system according to U.S. Pat. No. 5,686,728 has a projection objective with six mirrors, where each of the reflective mirror surfaces has an aspherical form. The mirrors are arranged along a common optical axis in such a way that an obscuration-free light path is achieved. Since the projection objective known from U.S. Pat. No. 5,686,728 is used only for UV light with a wavelength of 100-300 nm, the mirrors of this projection objective have a very high asphericity of approximately xc2x150 xcexcm as well as very large angles of incidence of approximately 38xc2x0. Even after reducing the image side aperture to NA=0.2, an asphericity of 25 xcexcm from peak to peak remains, with a barely reduced angle of incidence. Such asphericities and angles of incidence are not practicable in the EUV region due to the high requirements for surface quality and reflectivity of the mirrors.
Another disadvantage of the objectives disclosed in U.S. Pat. No. 5,686,728, which precludes their use with wavelengths below 100 nm such as the 11 nm and 13 nm wavelengths desirable for EUV microlithography, is the short distance between the wafer and the mirror arranged next to the wafer. In the case of U.S. Pat. No. 5,686,728, due to this short distance between the wafer and the mirror next to the wafer, the mirrors could be made only very thin. Due to the extreme layer stress in the multilayer systems discussed for 11 nm or 13 nm wavelengths, such thin mirrors are very unstable.
A projection objective with six mirrors for use in EUV lithography, particularly also for wavelengths of 13 nm and 11 nm, has become known from EP 779,528. This projection objective also has the disadvantage that at least two of the six mirrors have very high asphericities of 26 and 18.5 xcexcm. However, even in the EP 779,528 arrangement, the optical free working distance between the mirror next to the wafer and the wafer itself is so small that either instabilities occur or the mechanical free working distance is negative.
Thus, it is desirable to provide a projection objective for lithography with short wavelengths, preferably smaller than 100 nm, which does not have the disadvantages of the state of the art described above.
According to one aspect of the invention, the shortcomings of the prior art are overcome by a projection objective having an object plane and an image plane and a light path for a bundle of light rays from the object plane to the image plane. The six mirrors of the objective are arranged in the light path from the object plane to the image plane. According to the invention the mirror closest to the image plane where e.g. an object to be illuminated such as a wafer is situated is arranged in such a way that an image-side numerical aperture is NAxe2x89xa70.15. In this application the image-side numerical aperture is understood to be the numerical aperture of the bundle of light rays impinging onto the image plane. Furthermore, the mirror arranged closest to the image plane of the objective is arranged in such a way that the image-side free working distance corresponds at least to the used diameter of the mirror next to the wafer. In a preferred embodiment the image-side free working distance is at least the sum of one-third of the used diameter of the mirror next to the image plane and a length between 20 and 30 mm. In an alternative embodiment the image-side free working distance is at least 50 mm. In a particularly preferred embodiment, the image-side free working distance is 60 mm. In this application the free working distance is defined as the distance of the vertex of the surface of the mirror next to the image plane and the image plane. All surfaces of the six mirrors in this application are rotational-symmetric about a principal axis (PA). The vertex of a surface of a mirror is the intersection point of the surface of a mirror with the principal axis (PA). Each mirror has a mirror surface. The mirror surface is the physical mirror surface upon which the bundle of light rays traveling through the objective from the object plane to the image plane impinge. The physical mirror surface or the used area of a mirror can be an off-axis or an on-axis mirror segment relative to the principal axis (PA).
According to another aspect of the invention, a projection objective that comprises six mirrors is characterized by an image-side numerical aperture, NA, greater than 0.15 and an arc-shaped field width, W, at the wafer in the range 1.0 mmxe2x89xa6W. The peak-to-valley deviation, A, of the aspheres are limited with respect to the best fitting sphere of the physical mirror surface of all mirrors by:
Axe2x89xa619 xcexcmxe2x88x92102 xcexcm (0.25xe2x88x92NA)xe2x88x920.7 xcexcm/mm (2 mmxe2x88x92W).
In a preferred embodiment, the peak-to-valley distance A of the aspheres is limited with respect to the best fitting sphere of the off-axis segments of all mirrors by:
Axe2x89xa612 xcexcmxe2x88x9264 xcexcm (0.25xe2x88x92NA)xe2x88x920.3 xcexcm/mm (2 mmxe2x88x92W).
According to yet another aspect of the invention, a projection objective that includes six mirrors is characterized by an image-side numerical aperture NAxe2x89xa70.15 and an image-side width of the arc-shaped field Wxe2x89xa71 mm, and the angles of incidence AOI are limited for all rays of the light bundle impinging a physical mirror surface on all six mirrors S1, S2, S3, S4, S5, S6 by:
AOIxe2x89xa623xc2x0xe2x88x9235xc2x0(0.25xe2x88x92NA)xe2x88x920.2xc2x0/mm(2 mmxe2x88x92W)
wherein the angles of incidence AOI refer to the angle between the incident ray and the normal to the physical mirror surface at the point of incidence. The largest angle of any incident bundle of light rays occurring on any of the mirrors is always given by the angle of a bundle-limiting ray.
Preferably, an embodiment of the invention would encompass all three of the above aspects, e.g., an embodiment in which the free optical working distance would be more than 50 mm at NA=0.20 and the peak-to-valley deviation of the aspheres, as well as the angles of incidence, would lie in the regions defined above.
The asphericities herein refer to the peak-to-valley (PV) deviation, A, of the aspherical surfaces with respect to the best fitting sphere of the physical mirror surface of an specific mirror. The physical mirror surface of a specific mirror is also denoted as the used area of this specific mirror. The aspherical surfaces are approximated in the examples by using a sphere. The sphere has a center on the figure axis vertex of the mirror. The sphere intersects the asphere in the upper and lower endpoint of the used area in the meridian section. The data regarding the angles of incidence always refer to the angle between the incident ray and the normal to the physical mirror surface at the point of incidence. The largest angle of any incident bundle of light rays occurring on any of the physical mirror surfaces is always given by the angle of a bundle-limiting ray. The used diameter or the diameter of the physical mirror surface will be defined here and below as the envelope circle diameter of the physical mirror surface or the used area of a mirror, which is generally not circular.
In a preferred embodiment, the free working distance is 60 mm.
The objective can be used not only in the EUV, but also at other wavelengths, without deviating from the scope of the invention. In any respect, however, to avoid degradation of image quality, especially degradation due to central shading, the mirrors of the projection objectives should be arranged so that the light path of the bundle of light rays traveling from the object plane to the image plane is obscuration-free. Furthermore, to provide easy mounting and adjusting of the system, the physical mirror surfaces have a rotational symmetry to a principal axis (PA). Moreover, to have a compact design with an accessible aperture and to establish an obscuration-free light path of the bundle of light rays traveling from the object plane to the image plane, the projection objective device is designed in such a way that an intermediate image of the object situated in the object plane is formed after the fourth mirror. In such systems, it is possible that the aperture stop is situated in the front, low-aperture objective part, with a pupil plane conjugated to the aperture stop imaged in the focal plane of the last mirror. Such a system ensures telecentricity in the image plane.
In a preferred embodiment of the invention, the aperture stop is freely accessible and arranged in the light path from the object plane to the image plane between the second and third mirror. Good accessibility of the aperture stop is ensured when the ratio of the distance between the first and third mirror to the distance between the first and second mirror lies in the range of:
0.5 less than S1S3/S1S2 less than 2.
As defined for the free working distance in general a distance between two mirrors is the distance of the vertices of the surfaces of these mirrors.
Furthermore, in order to prevent vignetting of the light running from the third to the fourth mirror, by the aperture stop arranged between the second and third mirror, the ratio of the distance between the second mirror and aperture stop to the distance between the third mirror and the aperture stop lies in the range:
0.5 less than S2 aperture/(S3 aperture) less than 2.
In such a system, the angles of incidence on the physical mirror surfaces in the front part of the objective are reduced.
An aperture stop which physically lies between the second mirror, S2, and the first mirror, S1, must be formed at least partially as a narrow ring in order to avoid clipping of light moving from S1 to S2. In such a design, there is a danger that undesirable direct light or light reflected on S1 and S2, will pass outside the aperture ring and reach the image plane and thus the wafer. However, if the aperture stop is placed in the light path between the second and third mirror and physically close to the first mirror (which can be easily achieved mechanically), an efficient masking of this undesired light is possible. The aperture stop can be designed both as an opening in the first mirror or an opening which is arranged behind the first mirror.
In another embodiment of the invention, the aperture stop is arranged on or near the second mirror. Arrangement of the aperture on a mirror has the advantage that it is easier to manufacture.
In order to ensure an obscuration-free ray path with simultaneously low angles of incidence, the ratio of the distance between the first and third mirrors (S1S3) to the distance between the first and second mirrors (S1S2) lies in the range:
0.3xe2x89xa6S1S3/S1S2xe2x89xa62.0,
while the ratio of the distance between the second and third mirrors (S2S3) to the distance between the third and fourth mirrors (S3S4) lies in the range:
0.7xe2x89xa6S2S3/S3S4xe2x89xa61.4.
In order to be able to make the necessary corrections of imaging errors in the six-mirror systems, in a preferred embodiment, all six mirrors are designed to be aspherical. However, an alternative embodiment whereby at most five mirrors are aspherical can simplify the manufacturing, because it is then possible to design one mirror, preferably the largest mirror, i.e., the quaternary mirror, in the form of a spherical mirror. Moreover, it is preferred that the second to sixth mirror be in a concave-convex-concave-convex-concave sequence.
In order to achieve a resolution of at least 50 nm, the design part of the rms wavefront section of the system should be at most 0.07xcex and preferably 0.03 xcex.
Advantageously, in the embodiments of the invention, the objectives are always telecentric on the image-side.
In projection systems, which are operated with a reflection mask, a telecentric light path on the object-side is not possible without illumination through a beam splitter, which reduces the transmission strongly. One such device is known from JP 95 28 31 16.
In systems with transmission mask, the projection objective can be telecentric on the object side. In these embodiments, the first mirror is preferably concave.
The telecentericity error in the image plane, where the wafer is situated should not exceed 10 mrad and is typically between 5 mrad and 2 mrad, with 2 mrad being preferred. This ensures that changes of the imaging ratio remain within tolerable limits over the depth of focus.
In an preferred embodiments of the invention, the six mirror objective may include a field mirror, a reducing three-mirror subsystem and a two-mirror subsystem.
In addition to the projection objective according to the invention, the invention also makes available a projection exposure apparatus that includes at least a projection objective device. In a first embodiment, the projection exposure apparatus has a reflection mask, while, in an alternative embodiment, it has a transmission mask. Preferably, the projection exposure apparatus includes an illumination device for illuminating an off-axis arc-shaped field and the system is designed as an arc-shaped field scanner. Furthermore, the secant length of the scan slit is at least 26 mm and the ring width is greater than 0.5 mm.
The invention will be described below with the aid of the drawings as examples.